The use of catheters containing conducting elements in a Magnetic Resonance Imaging (MRI) environment poses severe safety problems, mainly due to RF heating effects. This prevents the practical use of many transcatheter interventional instruments and thus hampers interventional procedures under MRI guidance despite clear advantages of this imaging method like strong soft tissue contrast and the absence of ionizing radiation.
For a reliably robust visualization of the catheter in the MR image it is necessary to connect a receiving antenna (e.g. a small coil or a loopless antenna) at its tip with the receiver of the MR scanner. One way to realize this connection without causing dangerous RF heating is to implement transformers in the transmission line as described, for instance, in WO 2006/003566. The transformers block induced common mode currents, while differential mode currents like the MR signal can pass.
A way of adjusting the transformers using additional lumped element matching networks, comprising at least one T-, L- and/or π-quadrupole, which each comprise at least two impedance elements in the form of a capacitor and/or an inductivity, is described in WO 2005/103748.
For optimal signal transmission it is necessary to match the output impedance of any transformer in the transmission line to its complex conjugated value (power matching). This can be accomplished by using the matching networks as described in WO 2005/103748. In particular a network is used to match the output of the transformer to a transmission line having typically Z0=50Ω impedance. Then this line is matched at its end using another network to the input of the next transformer. The matching networks contain lumped elements, normally capacitors.
On the one hand side it is not easy to solder these matching networks due to the small size, which is required to fit into a catheter. On the other hand side every additional soldering joint is a possible source of malfunction, especially in a flexible catheter, where stress and torque can occur.